1. Field of the Invention
The present invention relates generally to connectors and more particularly to fully shielded, edge card connectors for use with capacitive proximity probes and like devices.
2. Description of Prior Art
The difficult measurement problem of determining the size of thin gaps in machinery and machine parts is presently addressed by such means as mechanical feeler gauges and thin capacitive proximity probes. Electronic measurement devices are preferred over mechanical means, since electronic measurement devices typically avoid the contact between the probe and the workpiece, often required by mechanical devices. Electronic devices also yield significantly higher measurement accuracy.
To achieve maximum sensitivity and range with a given probe size, capacitive probes may employ a shielded signal path from the sensing region of the probe to an apparatus for generating and receiving probe signals. Thus, one area of significant development in the capacitive probe art is that of providing adequate shielding of connections between the probe and associated cable. One capacitive probe of the prior art, which is shown in FIG. 1, receives signal and shield conductors in a soldered connection 100 to a shielded coaxial cable 107. However, the configuration of FIG. 1 does not provide any shielding of the exposed portions of the signal conductors 101, the signal trace 103 or below the sensing region 105. Coaxial cable 107 is provided with a braided shield 109 for shielding signal conductor 101 up to a region 110 where signal conductor 101 is exposed to permit formation of the soldered connection 100 between signal conductor 101 and plated through hole 111. Ambient noise signals may impinge on region 110, thus introducing noise, or region 11 may couple a signal carried on signal conductor 101 to an undesired point, because of the lack of shielding in region 110. Braided shield 109 is connected by a second solder joint to plated through hole 113 for forming a connection with shield trace 115. Shield trace 115 provides some shielding for the signal trace 103 and the sensing region 105 in a direction co planar with those traces. However, as described above, no shielding is provided below signal trace 103 or sensing region 105, or above signal trace 103. Thus, the above described prior art does not provide maximum sensitivity or accuracy, since signal leaked from signal trace 103, for example, will be interpreted as part of the signal at sensing region 105. Furthermore, ambient noise signals capacitively coupled into the probe from the outside environment may also contribute to measurement error.
FIG. 2 is an exploded view of an improved probe construction of the prior art which achieves 100% shielding at the desired frequencies of operation by providing some metallic shield conductor coverage over the entire signal path. In this construction, a laminate is formed of a series of insulating substrates 201, 203 and 205, each of which carries conductive traces. Substrate 201 carries a signal trace 103 and a sensing region 105. A solder connection between signal conductor 101 and a contact pad 219 is formed at an end of signal trace 103 opposite sensing region 105. Substrate 203 carries a shield trace 207 for shielding below signal trace 103, sensing region 105 and contact pad 219. Substrate 205 carries ring shield trace 209 for shielding above signal trace 103 and contact pad 219, while providing an unshielded aperture 21 above sensing region 105. Shield braid 109 is disposed in contact with and soldered to plated through hole 113, and further extends through hole 221 and is also soldered to shield trace 207. The entire assembly is laminated together and affixed to a carrier substrate 213. A protective cover 215 may also be provided and affixed over the assembly. Thus the shield braid 109 in connection with shield layers 209 and 207 above and below the signal trace 103 assure a continuous shield everywhere but the aperture 210 above the sensing region 105.
Thin probes are desirable in order to measure the size of extremely narrow openings, such as the spacing between adjacent parts of a machine. However, extremely thin probes are subject to wear or breakage. Thus, in order to facilitate replacement of a worn probe by a user, the probe is connected to the measurement apparatus through a cable 107 having a connector 217 at one end.
The cable 107 is generally of a thin, flexible coaxial type. A miniature coaxial connector 217 is provided for connecting the probe assembly to measurement equipment (not shown). However, miniature coaxial connectors presently available are relatively expensive and along with the cable typically form a substantial portion of the selling price of a finished probe. Furthermore, the cable and connector are permanently attached to the probe body by the permanent solder joints. The cost of direct labor to construct the joint is another substantial portion of the selling price of probes of the prior art.
Although capacitive probes are typically used in conjunction with non-contact techniques, they are subject to wear and accidental breakage from constant handling and some frictional abrasion, as noted above. Thus, an expensive probe assembly is costly to the consumer, who must maintain equipment including these probes. The consumer desires an inexpensive probe which is easy to replace. Furthermore, the consumer would prefer to move any cost burden away from replaceable components of a system, in favor of the fixed components of a system.
Further, capacitive probes of the prior art that have been illustrated and described thus far include only one sensing region 105 and signal trace 103; however probes are known which include multiple sensing regions, each fed by a separate signal wire. In such a device, the number of cables, connectors and solder joints which constitute the more expensive portions of the probe are multiplied accordingly. For example, in a probe including two sensing regions, each fed by a separate signal wire, two cables and connectors would typically be used. Thus, in this example, the high cost to the consumer discussed above is doubled.
Accordingly, it is a general aim of the present invention to provide an inexpensive, shielded connection from one or more signal conductors in one or more shielded cables to a thin, flat circuit board, which may be subjected to replacement, such as a capacitive probe.